1. Field of the Invention
The present invention relates to a motor speed control circuit.
2. Description of the Related Art
Various electronic apparatuses have an exothermic body that generates heat when the electronic apparatus operates. To cool this exothermic body, a fan motor is usually provided. For example, in personal computers, servers and the like, the operating frequencies of the CPUs become increasingly high year by year causing the heat values of the CPUs to increase. Accordingly, a fan motor for cooling the CPU and a motor driver to drive the fan motor is usually provided in personal computers, servers and the like. As a speed control method for fan motors, a speed servo control method has been proposed which has a PWM drive method combined therewith as shown, e.g., in FIG. 10 (refer, for example, to Japanese Patent Application Laid-Open Publication No. 2003-204692).
To describe in detail, a rotational speed detection signal obtained from a pulse generator PG of a motor 1 is supplied to an operational amplifier 7 for generating a speed voltage. The output of this operational amplifier 7 is integrated by an RC filter circuit to produce a direct-current speed voltage VV, which is applied to the inverting input terminal of a comparator 9. Furthermore, a PWM (Pulse Width Modulation) signal set by a CPU 5 is supplied to an operational amplifier 6 for generating a reference voltage. The PWM signal sets the rotational speed of the motor 1 via its duty ratio. The output of the operational amplifier 6 is integrated by an RC filter circuit to produce a direct-current reference voltage VR1, which is applied to the non-inverting input terminal of the comparator 9.
The comparator 9 compares the speed voltage VV applied to the inverting input terminal and the reference voltage VR1 applied to the non-inverting input terminal, and produces and outputs a control signal VC as the comparing result. A motor driver 11 causes the amount of current corresponding to the control signal VC from the comparator 9 to flow through the drive coil of the motor 1 thereby controlling the rotational speed of the motor 1. Furthermore, a hall element 13 is provided for the stator of the motor 1, and the motor driver 11 controls the rotational direction of the motor 1 by switching the direction of the current flowing through the drive coil of the motor 1 on the basis of the hall element output of the hall element 13 indicating the detected position of the rotor.
As such, in order to perform speed servo control of the fan motor, as shown in FIG. 10, there is usually provided circuitry that is equivalent to the operational amplifier 7 that generates the speed voltage VV indicating the detected, actual rotational speed of the motor 1, the operational amplifier 6 that generates the reference voltage VR1 of a level according to a motor rotational speed-specifying signal such as the PWM signal, and the comparator 9 that compares the speed voltage VV supplied from the operational amplifier 7 and the reference voltage VR1 supplied from the operational amplifier 6.
The comparator 9 is usually an operational amplifier in configuration as shown in FIG. 10. To describe in detail, the comparator 9 comprises a differential transistor pair (T1, T2), a constant current source T3 provided on the ground voltage GND side of the differential transistor pair (T1, T2), and a current mirror circuit (T4, T5) provided on the bias voltage VREG side of the differential transistor pair (T1, T2). That is, the current mirror circuit (T4, T5), the differential transistor pair (T1, T2), and the constant current source T3 are connected in series between the bias voltage VREG and the ground voltage GND. Note that depending on the type of transistors, the constant current source T3 may be provided on the bias voltage VREG side of the differential transistor pair (T1, T2) and that the current mirror circuit (T4, T5) may be provided on the ground voltage GND side of the differential transistor pair (T1, T2).
The problem to be solved by the invention will be explained below using as an example the motor speed control system shown in FIG. 10 with reference to FIG. 11.
The non-inverting and inverting inputs of the comparator 9 are connected to the base electrodes of the differential transistor pair (T1, T2). The current mirror circuit (T4, T5) is provided on the bias voltage VREG side of the differential transistor pair (T1, T2) and the constant current source T3 is provided on the ground voltage GND side of the differential transistor pair (T1, T2). In this case, the allowable range of voltage to be applied between the non-inverting and inverting inputs of the comparator 9 is supposed to be ideally the range of from the bias voltage VREG to the ground voltage GND.
However, the upper level limit for the non-inverting and inverting inputs of the comparator 9 is lower by at least a collector-to-emitter saturation voltage VCE(sat) of the current mirror circuit (T4, T5) than the bias voltage VREG. Also, the lower level limit for the non-inverting and inverting inputs of the comparator 9 is higher by at least a collector-to-emitter saturation voltage VCE(sat) of the constant current source T3 than the ground voltage GND. As a result, the allowable range of voltage to be applied between the non-inverting and inverting inputs of the comparator 9 is limited. This limited, applied voltage range is generally called a common-mode input voltage range.
Here, on the premise that the comparator 9 has the common-mode input voltage range as an electrical characteristic, consider a case where the duty ratio of the PWM signal is varied from 0% to 100%. In this case, the reference voltage VR1 applied to the non-inverting input of the comparator 9 varies in level approximately from the ground voltage GND to the bias voltage VREG in response to the change in the duty ratio of the PWM signal. However, since being subject to limitation of the common-mode input voltage range, the non-inverting input of the comparator 9 cannot respond to all changes in the reference voltage VR1 corresponding to the change in the duty ratio of the PWM signal. That is, there is the problem that the range of from 0% to 100% for the PWM signal duty ratio is partly outside the common-mode input voltage range as an electrical characteristic of the comparator 9 and in this case, the comparator 9 does not operate correctly.